1. Field of the Invention
The present invention relates to a system for determining true north heading information. More particularly, the present invention is directed to a system which employs an inertial sensor system and a satellite positioning system for relating the inertial sensor system reference frame to a geographic reference frame.
2. Description of the Related Art
Global positioning systems incorporating the use of satellites are well-known in the art and are sometimes referred to as "satellite positioning systems". Such systems, for example, NAVSTAR-GPS (or simply GPS), are accurately being used for the determination of the geocentric position of mobile units, for example, among others, land vehicles, aircraft and survey equipment. Satellite positioning systems, herein referred to as "GPS systems", are now being employed for aircraft navigation and flight control, and airspace control. These GPS systems may operate independently or in combination with inertial sensor systems for providing aircraft attitude information during an aircraft flight mission.
As is well-known, the federal aeronautical administration requires aircraft to employ some system for determining true north heading, i.e., the heading of the aircraft relative to "true north". One such system employs the use of an AHRS (Attitude Heading Reference System) system in combination with a magnetic flux valve from which magnetic north is determined and true north is derived from magnetic deviation tables. Although an AHRS system commonly employs sufficient sensors--namely, three gyros and three accelerometers--to provide aircraft attitude information, the gyros may not have sufficient accuracy to provide accurate heading information. Generally speaking, an AHRS system is a "low performance grade" inertial navigational system which is sufficient to provide accurate pitch and roll information, but is insufficient, by itself, to provide a reliable indication of true north heading. To overcome deficiencies of the AHRS system alone, magnetic flux valve readings are blended in a filter with the AHRS attitude information so as to provide a more accurate magnetic north heading angle. In turn, this later angle may be corrected by use of the magnetic deviation look-up table for a precise determination of the true north heading angle of the aircraft.
In contrast, use of GPS position and velocity information alone may be used for purposes of determining true north heading. Although, the GPS information may be reliable for a static condition--either a non-moving mobile unit or a very slowing moving unit, it is insufficient, by itself, to provide accurate heading information for an aircraft whose mobile position is rapidly and dynamically changing.